Proximity detection system for deep wells

ABSTRACT

A method and apparatus for magnetic field measurements to determine the proximity of a nearby target incorporating electrically conductive material includes a drill string ( 54 ) having multiple drill pipe sections ( 56, 57, 58, 59 ) connected end-to-end, with at least one of the drill pipe sections ( 57 ) being electrically conductive and isolated to provide an electrode section. A nonmagnetic drill pipe section ( 84 ) is connected in the drill string below the electrode section ( 57 ), and a hydraulic motor ( 62 ) having a rotatable drill bit sub ( 70 ) carrying a magnetic field sensing instrument package ( 102 ) is connected to a lowermost end of the drill string. A power supply provides a time-varying current to the drill pipe electrode section ( 57 ) to produce a corresponding target current magnetic field to be detected at the drill bit instrument ( 10 ) 2 , and a communication instrument package ( 94 ) is locatable within the nonmagnetic drill pipe section ( 84 ) to receive magnetic field data from the magnetic field sensing instrument package ( 102 ) on the drill bit ( 70 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-in-Part of U.S. application Ser. No.12/342,034, filed Dec. 22, 2008, and entitled “Wireline CommunicationSystem for Deep Wells”, the entire disclosure of which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates, in general, to methods and apparatus forlocating a nearby conductive target, such as a cased well or borehole,from a remote location such as a second borehole or deep well to obtaindata for use in guiding the direction of drilling the second well withrespect to the target. More particularly, the invention is directed tomethods and apparatus for injecting time-varying electrical currentsinto the earth from one or more electrodes in a borehole being drilled,for detecting at the drill bit of that borehole the electromagneticfield vectors resulting from the portion of such injected currents whichflows in metal at the target, and for transmitting data representing thedetected fields to the earth's surface for use in determining theproximity of the target.

BACKGROUND OF THE INVENTION

It is well known that in drilling boreholes in the earth, such as deepwells for oil and gas exploration, precise control of the path followedby the well is extremely difficult, so that it is virtually impossibleto know the exact location of the well at a given depth. For example, adrilling tolerance of plus or minus one quarter of a degree will allowthe bottom of a 10,000-foot well to be positioned anywhere within acircle 100 feet in diameter, and numerous other factors can increase thedeviation. This is not of particular concern in many drillingoperations, but if drilling precision is necessary, as where a boreholeis to be drilled precisely to intersect a target location, is to bedrilled so as to avoid an existing well, or is to be drilled to beparallel to an existing borehole, such variations can cause severedifficulties. One example of the need for precision drilling occurs inthe situation where it becomes necessary to drill a relief well tointersect an existing deep well, as in the case where the casing of thedeep well has ruptured and it becomes necessary to plug the well at orbelow the point of the rupture to bring it under control. In order to dothis, the relief well must be drilled to intersect the original well atthe desired level, and since such ruptures, or blowouts, often produceextremely hazardous conditions at the surface in the vicinity of theoriginal well, the relief well usually must be started a considerabledistance away from the original wellhead and drilled at an incline downto the desired point of intersection.

Because the same problems of control of the direction of drilling thatwere encountered in the original well are also encountered in drillingthe relief well, the location of the relief well borehole also cannotalways be known with precision as the relief well is being drilled;accordingly, it is extremely difficult to determine the distance anddirection from the end of the relief well to the desired point ofintersection on the target well. In addition, the relief well usually isvery complex, compounding the problem of knowing exactly where it islocated with respect to a target that may be 10 inches in diameter at adistance of thousands of feet below the earth's surface.

Numerous early attempts were made to solve the problem of guiding arelief well to accurately intersect a target well. Some utilizedsurveying techniques to locate the relief well with respect to a targetwell, but such survey techniques are not capable of providing accuratedata concerning the relationship of the relief well to the original welluntil the relief well has approached very near the original well.Magnetic gradient ranging equipment can be used with considerableaccuracy at close range; however, it has been found that outside aradius of a few tens of feet, such systems are usually inadequate.

In an attempt to extend the distance at which accurate information canbe obtained, a variety of electrical well logging techniques have beenused which treat the target well as an anomaly in the geologic structureof the earth surrounding the relief well. Some of these systems aredirected to the measurement of the apparent resistivity of the earthacross a pair of electrodes but, since no directionality is given bythis method, it is ineffective for directing a relief well with respectto an existing well.

In addition, there have been attempts to obtain similar data through theuse of electromagnetic prospecting, where induction sensing coilsmounted at right angles to each other are used in conjunction with otherconventional well logging systems to determine the probable location ofa target. However, such systems do not suggest the possibility oflocating relatively small targets such as well bores.

Other systems have been developed for directing a second well withrespect to a first well by the use of sonic detectors responsive to thesound produced by fluids flowing out of a ruptured well formation.However, such systems do not operate when there is no sound emanatingfrom the target well, and, in addition, do not provide the requireddegree of directional and distance accuracy. Another system uses asignal transmitter in one well and a signal receiver in the other well,wherein sound waves or magnetic fields may be used as the signals. Insuch a system, however, the target well must be accessible so that thesignal source can be placed in one well and the receiver in the other,and is not effective where the target well is not open.

Many of the difficulties outlined above were overcome in the prior artby methods and apparatus disclosed, for example, in U.S. Pat. Nos.4,323,848, 4,372,398, 4,700,142, and 5,512,830, all issued to Arthur F.Kuckes, the applicant herein. In accordance with these patents, anelectric current flow is produced in a target such as the casing of atarget well by injecting a low frequency alternating current into theearth surrounding the target well through the use of an electrodelocated in a relief well, or borehole. This current flow extends betweenthe downhole electrode and a second electrode that may be located at theearth's surface in the vicinity of the head of the relief well. Aportion of the injected earth current finds a path of least resistancethrough the casing or other current-conducting material in the targetborehole, and the resulting concentration of current produces acharacteristic magnetic field surrounding the target well which can bedetected by an AC magnetic field sensor such as that described in U.S.Pat. No. 4,323,848, or by multiple sensors, as described in U.S. Pat.No. 5,512,830. These sensors are extremely sensitive to very smallmagnetic fields, and accurately detect the vectors of magnetic fieldsproduced by currents flowing in well casings located a considerabledistance away from the relief borehole.

The vector signals obtained from the AC magnetic field sensors, inaccordance with the aforesaid patents, permit calculation of thedirection and distance to the target well casing with respect to thelocation of the AC magnetic field sensor in the relief well. Thisinformation can be used to guide further drilling of the relief well.Thus, as the relief well approaches a desired depth, its approach to thelocation of the target well can be guided so that the target well isintersected at the desired depth below the earth's surface in a rapidand effective manner. This method of guiding a relief well to intersectwith a target is a homing-in process, wherein multiplemeasurements—often after every 50 feet of drilling—are made as therelief borehole approaches the target. Since the drill string for therelief well must be pulled for each measurement, the drilling of arelief well becomes very expensive, especially in off-shore drilling,wherein more time may be spent measuring than is spent drilling.

The foregoing systems are widely, and successfully, used; however, theneed for time-consuming periodic withdrawals of the drill string so thatsuitable sensors and electrodes for generating the ground current can belowered into place to obtain distance and direction measurements fromthe relief well is a drawback, since a drilling rig operation can costupwards of $500,000.00 per day in offshore drilling operations.Accordingly, a method and apparatus for making such measurements withoutthe effort and expense of pulling the drill string is needed.

Another difficulty encountered in typical borehole drilling operationssuch as those described above is that the path of the borehole, whichmay be a relief well as described above, is tracked during drilling by a“measurement while drilling” (MWD) instrument that is mounted near, butnot at, the bottom of the drill string. Usually, a drill string consistsof a series of steel tubes, each about 10 meters in length and connectedend-to-end. Connected near the bottom end of the drill string is anon-magnetic section which carries the MWD instrument, and connectedbelow that is a hydraulic drilling motor having a bent housing to whichthe drill bit is connected via a drill shaft, with each of thenon-magnetic section and the bent housing being about 10 meters inlength. As a result of this, the MWD instrument is typically located10-20 meters above the face of the drill bit, so that when magneticfield measurements are made with the drill string in the relief well,they are actually made a considerable distance from the drill bit,introducing a significant error in determination of the relativedistance and direction of the target with respect to the drill bit. Thisgreatly increases the difficulty of accurately controlling the locationof the borehole being drilled with respect to the target.

Accordingly, there is a need for a measurement system that willsignificantly increase the accuracy of distance and directioncalculations in drilling, without withdrawing the drill string from theborehole being drilled.

SUMMARY OF THE INVENTION

Briefly, the present invention is directed to an improved method andapparatus for determining the distance and direction from the drill bitof a drill string in a borehole being drilled to a target location, suchas the center of an existing borehole casing, without the need towithdraw the drill string to make the necessary measurements, by makingmagnetic field measurements from a drill bit sub, or drill head at thebottom of the relief well and communicating those measurements to thesurface so that accurate calculations can be made.

In accordance with one aspect of the present invention, the need forpulling a drill string in order to make magnetic field measurements in arelief well, or borehole, is obviated by the use of magnetic fieldsensors mounted in a drill bit instrument that is secured to the drillbit sub, in combination with a drill string wireline having a suitablecurrent-injecting electrode and a wireline instrument package which canbe dropped down through the center of the drill string whenever ameasurement is to be made. The electrode is energized with atime-varying current that is injected into the earth to produce acorresponding magnetic field generated by current flow in the target,and the drill bit instrument detects that magnetic field at the drillbit. The drill bit instrument transmits data representing measured fieldvectors, and the wireline instrument package receives that data andtransmits it to the surface for use in guiding further drilling. Thewireline is then withdrawn, and drilling can be resumed.

The foregoing process is carried out, in accordance with another aspectof the invention, through the use of a modified drill string structurehaving at least one insulating segment, but preferably two suchsegments, spaced apart to electrically isolate a selected conventionaltubular, electrically conductive, steel drill string pipe section nearthe bottom of the string. This electrically isolated section forms adrill string electrode that is located along the drill string forelectrical communication with the above-described wireline electrode.Generally, drill string pipe sections are about ten meters in length andare joined end-to-end, with sections being added to the drill string asdrilling progresses. In accordance with the invention, each insulatingsegment, or sub, is about one meter in length, with a single subgenerally being sufficient for electrical isolation of an electrodesection, although additional insulating subs may be used, as needed. Thedrill string preferably includes a single such electrode section,although in some circumstances it may be desirable to include two spacedelectrode sections separated and isolated from each other by at leastone insulating sub. If desired, they may be spaced further apart byincluding one or more non-electrode steel pipe sections between theinsulating subs for the upper and lower electrode sections. The modifieddrill string includes a nonmagnetic segment, in which is mounted aconventional MWD instrument, and a lowermost (distal) end segment whichis a standard rotating drill bit carried in a drill head, or sub, thatis connected to a standard directional drilling assembly which mayutilize, for example, a hydraulic drilling motor incorporating a benthousing for directional drilling control, in known manner. As is known,such a hydraulic drilling motor may be driven by drilling fluid thatflows down the center of the drill string and back up the boreholeoutside the string. It will be understood that although the presentinvention is described in a rotary drilling assembly which utilizes ahydraulic drilling motor, such an assembly is only one of many knownways to control the directional drilling of a borehole, and that thepresent invention may be used in other such drilling assemblies. Inaccordance with this embodiment of the invention, the drill bit subcarries a magnetic field sensor instrument which incorporates detectorsfor measuring magnetic fields in the vicinity of the drill bit, and inparticular for detecting the vectors of the characteristic magneticfields generated by current flow in the target.

In this embodiment of the invention, when a magnetic field measurementis to be made using the drill string of the invention, drilling ishalted, but instead of withdrawing the drill string, a wireline carryinga wireline electrode and a data receiver and transmitter instrument islowered through the center of the drill string until the wirelineelectrode is aligned with the approximate center of the correspondingisolated steel drill pipe electrode section and the transmitter/receiverinstrument is located in the nonmagnetic section of the drill string,below the electrode. The wireline electrode is in electricalcommunication with its corresponding isolated steel drill pipe electrodesection which is, in turn, in electrical communication with thesurrounding earth formations. When the wireline electrode is energized,the drill pipe electrode injects current from the wireline electrodeinto the surrounding formations and a portion of that current is thencollected in the target. The electrodes are energized by a periodictime-varying current, such as a sinusoidal AC supplied from a powersupply at the earth's surface, to produce a characteristic targetcurrent and a corresponding characteristic target magnetic field. Thewireline electrode is immersed in the drilling fluid in the drillstring, and this fluid may be electrically conductive to provideelectrical communication between the wireline electrode and itscorresponding drill pipe electrode. In the case where a non-conductivedrilling fluid is used, spring-loaded contacts may be employed on thewireline electrode to provide a positive electrical contact with theinner surface of the isolated steel drill pipe section.

In accordance with the this embodiment of the invention, the desiredmagnetic field measurements are made at the drill bit sensor that islocated in the drill bit instrument package described above, and whichcontains the magnetic field detectors. This location for the drill bitsensor is advantageous, because it is at the location of the drill bitthat is to be controlled. The drill bit instrument is battery-operated,and in addition to suitable magnetic field vector detectors it includesgravity vector detectors and may include static magnetic field sensors.This drill bit instrument also incorporates suitable communicationelectronics, such as an electromagnetic solenoid, for transmitting datafrom the drill bit sensor instrument to the wireline instrument in thedrill string. The wireline instrument also includes suitablecommunication electronics to remotely receive the data from the drillbit sensor and to transmit that data to the surface either directly orvia the conventional data communication system incorporated in the MWDinstrument in the drill string.

In a second embodiment of the invention, the drill string incorporatesat least one, and preferably two spaced electrode sections that areelectrically isolated from the remainder of the drill string, includes aconventional MWD instrument mounted above a directional drillingsteering assembly which may incorporate a hydraulic drive motor, asdiscussed above, and includes a drill head mounted on a bent sub anddriven by the motor, with a magnetic field sensor instrument mounted onthe drill head. In this embodiment, the need for a wireline that can belowered down the center of the drill stem is eliminated. In its place isa downhole instrument package incorporating suitable telemetry such astransmit/receive communications electronics and located in a nonmagneticsection of the drill string for communicating with the drill headinstrument. This instrument package also includes a down-hole electrodepower supply such as a battery pack, an alternator, or both, with thepower supply being connected through suitable cables to the drill stringelectrode or electrodes for current injection from the down-hole powersource. The drill head sensor instrument includes suitable telemetrysuch as transmitter and receiver electronics for communicating with thetransmit/receive electronics in the down-hole instrument package, which,in turn, communicates with a standard MWD instrument that is alsolocated in the nonmagnetic section. The MWD instrument communicates withsurface receivers in conventional manner, as by way of pressure pulsesin the drilling fluid, to enable data transfer and control signals topass between instrumentation at the surface and the drill head sensorinstrument.

In another embodiment of the invention, magnetic field measurementaccuracy may be improved in some circumstances by operating the systemin a pulsed transient mode, wherein the earth formations surrounding therelief and the target wells are energized by a stepped, or pulsed,primary excitation current from a power source, which may be either atthe surface or downhole. Measurements of magnetic fields produced by theresulting current flow in the target are made at the drill head sensorinstrument immediately following a stepwise turn-off of the excitationcurrent, when that current is zero. Each pulse of electrical energysupplied to the electrodes causes a current to flow through the earth'sformations to the target, and, as described in the foregoing U.S. Pat.No. 4,700,142, this current is collected on the electrically conductivetarget. The resulting target current flow creates a characteristictarget magnetic field that is detected by the drill head sensorinstrument. In the pulsed, or transient, mode of operation of thisembodiment, the magnetic field measurement is made after the primaryenergizing current stops, so that the magnetic fields that are measuredwill be entirely due to eddy currents. That is, when the excitationcurrent is stepped to zero, measurements are made of the magnetic fieldsproduced by a decaying target well current flow due to induction effectsin the target/earth system. Although this decay current produces only avery small field, since even the primary target current typically isonly a few percent of the energizing current, the measurement of thedecay field is more accurate, since interfering fields caused by theprimary electrode current in the earth are not present. Although use ofa single downhole drill string electrode is feasible, theabove-described transient pulsed current magnetic field measurementbenefits greatly from the use of at least two vertically spaced,electrically isolated conductive drill string pipe electrode sections,each separated from each other and other adjoining pipe sections by oneor more electrically insulating subs.

Deep well measurements are made by supplying a time-variable AC currentof several amperes at about 10 Hertz, preferably to a pair of isolateddrill pipe electrode sections which effectively provide two drill pipeinjection electrodes spaced along the drill string above the drillmotor. The time-variable current supplied to the electrodes injects acorresponding current into the earth and produces a correspondingtime-varying target current. The vectors of the resulting characteristictarget magnetic field are detected at the location of the drill bit sub.Telemetry at the drill bit sub transmits the detected vector data upholevia a wireline instrument and/or a conventional MWD instrument for usein calculating the distance and direction from the drill bit sub to thetarget, and receives control signals from the surface instrumentation.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill be understood by those of skill in the art from the followingdetailed description of preferred embodiments thereof, taken with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic illustration of a prior art electromagnetictarget location system;

FIG. 2 is a graph illustrating target current flow amplitude in thesystem of FIG. 1;

FIG. 3 is a diagrammatic illustration of a wire line electrode system inaccordance with a first embodiment of the present invention,illustrating typical dimensions;

FIG. 4 is a schematic diagram of a drill bit sub sensor instrument inaccordance with the present invention;

FIG. 5 is a top sectional view of a drill bit sub and included sensorinstrument in accordance with the invention, taken along lines A-A ofFIG. 6;

FIG. 6 is a cross-sectional view of the drill bit sub and sensor, takenalong lines B-B of FIG. 5;

FIG. 7 is a diagrammatic illustration of a wireline instrument suitablefor use in the system of FIG. 3;

FIG. 8 is a diagrammatic illustration of formation and target currentflow for a single-electrode system of the present invention;

FIG. 9 is a diagrammatic illustration of a modified version of the wireline electrode system of FIG. 3, in which two vertically spaced drillstring electrodes are provided;

FIG. 10 is a diagrammatic illustration of a second embodiment of thepresent invention, incorporating drill string electrodes connected to adownhole power supply;

FIG. 11 is a diagrammatic illustration of the transmit/receive packageand power supply of the embodiment of FIG. 10;

FIG. 12( a) illustrates a drill string electrode excitation currenthaving a transient excitation waveform, in accordance with anotherembodiment of the invention, while FIG. 12( b) illustrates acorresponding target waveform caused by the excitation current of FIG.12( a) and exhibiting a transient decay due to eddy currents;

FIG. 14 illustrates magnetic fields generated in a surrounding earthformation by a decaying target current flow; and

FIG. 14 illustrates decaying current flow amplitudes in a target.

DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now to a more detailed consideration of the present invention,FIG. 1 illustrates, in diagrammatic form, a standard well locatingsystem 10 such as that described in U.S. Pat. No. 4,700,142, thedisclosure of which is hereby incorporated herein by reference. In sucha system, a target well 12 is to be located by drilling a second well,or borehole 14 along a path that will approach the target at a desireddepth below the earth's surface 16. As is known, the second borehole maybe a rescue well that may be intended to intersect a target, may beintended to locate and avoid the target, or may be intended to locateand then be drilled along a path that is parallel the target. Forsimplicity, such a second borehole may be referred to herein as a reliefwell, it being understood that it may be drilled for any purpose.Typically the target is a cased well or has a drill string or otherelectrically conductive material in it, so that electrical currentflowing in the earth's formations 18 surrounding the well 12 will tendto be concentrated on that conductive material. An alternatingelectrical current is injected into the earth by an electrode 20 carriedby a logging cable, or wireline 22, which is lowered into the reliefborehole 14 after the drill string that is used to drill the reliefborehole has been pulled out. The electrode is connected throughwireline 22 to one side of an AC source 24, the other side of which isgrounded at 26 to the earth 16. The electrode 20 contacts the uncasedsides of the relief well so that current from source 24 is injected intothe earth formations 18, as illustrated by arrows 30.

This injected current, which returns to the grounded side of thegenerator at 26, finds a path of least resistance through the casing orother conductive material in target well 12, producing a target currentflow indicated by arrows 32 and 34, respectively, above and below thedepth of the electrode 20. The upward current flow of current 32 isillustrated in FIG. 2 by curve 32′, while the downward flow of targetwell current 34 is illustrated in FIG. 2 by curve 34′. As illustrated,at the depth of the electrode equal and opposite currents on the targetproduce a net zero target current, while above and below that point thetarget currents maximize and then decline due to leakage into thesurrounding formation, as illustrated in FIG. 2, with these target wellcurrents eventually returning to the ground point 26 through the earth.

The concentrated current flow on the target well produces, for thedownward current 34, for example, a corresponding AC magnetic field 36in the earth surrounding the target well. This target AC field isdetectable by an AC field sensor, or sonde, 40 that is suspended in therelief well 14 by the wireline 22. The sonde 40, which preferably islocated below the electrode 20, incorporates suitable field componentdetectors, such as three orthogonal magnetometers, to measure the vectorcomponents of magnetic field 36 and to produce corresponding datasignals that are transmitted via the wireline to, for example, acomputer 42 at the surface.

Vector signals obtained from the magnetometers in the sensor 40,together with measurements of other parameters such as the orientationof the sensor, permit calculation of the direction and distance of thetarget well casing from the sensor, as described, for example, in U.S.Pat. No. 4,700,142 or 5,512,830. In the course of drilling the reliefwell, the drill string is withdrawn periodically and the wireline islowered into the relief borehole so that vector measurements andmeasurements of the orientation of the sensor within the borehole can bemade, and these, together with measurements of the relief well directionmade either at the same time or from previously made borehole surveydata, permit a continuous calculation of the presumed location of thetarget well with respect to the location of the relief well. Thewireline is then withdrawn and the drill reinserted into the reliefwell, and the calculated information is used to guide further drillingof the relief well. As the relief well approaches the desired depth, itsapproach to the location of the target well can be guided so that thetarget well is intersected at the desired depth below the earth'ssurface.

As discussed above, such prior systems require the withdrawal of thedrill string from the relief well in order to measure the targetmagnetic field. A preferred form of the improved system of the presentinvention allows target field measurements without requiring thewithdrawal of the relief drill string, and is illustrated at 50 in FIG.3, to which reference is now made. In accordance with this firstembodiment of the invention, a relief borehole, or well 52, which isillustrated in dashed lines, is produced by a drill carried by a drillstring 54 which is suspended from a surface drilling rig (not shown), inconventional manner. Such a drill string typically consists of multipledrill string sections of steel pipe, such as the illustrated sections56, 57, 58 . . . 59, each normally about ten meters in length andcoupled together end-to-end at threaded joints. In conventional manner,the bottom, or distal end of the drill string incorporates a standardrotary steering assembly such as the illustrated hydraulic drillingmotor 62 in a bent housing 64, with the motor having a rotating driveshaft 66 connected to a drill bit 68. In accordance with the invention,the drill bit is carried by a drill bit head, or instrument sub 70, tobe described in detail below, which rotates with the drill bit. Locatedin the drill string 54 just above the housing 64 is a conventionalmeasurement-while-drilling (MWD) measurement instrument for use inproducing a log of the drilling and for controlling the direction ofdrilling.

In accordance with one embodiment of the invention, at least one of theelectrically conductive drill pipe sections; for example section 57, iselectrically isolated from adjacent drill pipe sections to form a pipeelectrode for use in injecting current into the surrounding earthformations. This pipe electrode 57 is formed by inserting one or moreelectrically insulating subs 71 and 72, which may be short insulatingpipe sections about one meter in length, in the drill string above andbelow the drill pipe section 57 that is to be isolated, as illustratedin FIG. 3. The insulating sub 71 is threaded to the bottom of standardsteel pipe section 56 at threaded joint 74, and to the top of standardsteel pipe section 57, at threaded joint 76, to space and electricallyinsulate the adjacent pipe sections 56 and 57 from each other. Thesecond insulating sub 72 is threaded to the bottom of the steel drillpipe section 57 at threaded joint 78 and to the top of the next adjacentsteel drill pipe section 58 at threaded joint 80. Sub 72 separates, andelectrically insulates, adjacent steel pipe sections 57 and 58 from eachother, thereby electrically isolating pipe electrode section 57 from theremainder of the drill string.

Although a single insulating sub is shown at each end of pipe section57, it will be understood that multiple insulating subs may be used ateach location to improve the isolation of pipe 57, as needed, or theinsulating subs may be omitted if the resulting degraded performance isacceptable. The system will work without the upper insulated sub 71because the “easy way” to return to the surface is through the earth,rather than along the walls of the drill pipes. The lower insulated sub72 is very desirable, however, for even very tiny stray currents in thedrill pipes in the vicinity of the sensor instruments (to be described)will seriously degrade the operation of the system.

Connected below the isolated drill pipe electrode section 57 are one ormore additional steel drill pipe sections such as sections 58 . . . 59,the number of drill pipe sections being selected to position theelectrode section 57 at a desired distance above the drill bit. Asuitable distance between the pipe electrode 57 and the drill bit 68 maybe about 70 meters.

The lowermost end of the bottom drill pipe 59 preferably is connected ata threaded joint 81 through an electrically insulating sub 82 and athreaded joint 83 to a nonmagnetic drill pipe section 84, the lower endof which is connected at threaded joint 86 to the top of the directionaldrilling steering assembly housing 64. A standard MWD instrument in anMWD housing 88 preferably is located within the nonmagnetic pipe section84 in conventional manner for controlling the operation of the steeringassembly in conventional manner.

Locatable within the drill string 54 is a wireline 90, which issuspended from the earth's surface at the drill rig. During pauses inthe drilling operation, the wireline is lowered into the relief welldown through a central, axially-extending opening 91 in the drillstring. The drilling fluid flows through this axial opening, and when ahydraulic drive motor is used in the steering assembly to drive themotor 64, the central opening effectively terminates at the top of themotor. The wireline incorporates both a power cable for injecting ACcurrent into the earth and a data cable for connecting down-holeinstruments with the surface, and is covered by an insulating materialsuch as an electrically insulating layer of a plastic such as Hytrel forprotection from the harsh environment. The power cable in the wirelineis connected at the surface to a suitable source 24 (FIG. 1) of aperiodically varying current such as a low-frequency AC to producealternating current 96 in the cable, and is connected at its lower endto an electrode 92 which is uninsulated and is located on the wirelinefor electrical communication with the interior of the isolated drillpipe section 57. This electrode may physically contact the interior ofsection 57 by way of spring-loaded contacts, for example, although agood electrical connection can be made through the drilling fluid, ordrilling mud, if it is electrically conductive, since this fluid remainswithin the drill string during this process. Many modern drilling fluidsare a non-conductive synthetic material that is approximately 60% oiland 40% water, however, so a mechanical contact between the wirelineelectrode and the drill pipe may be preferred. The electrode 92 isaccurately locatable centrally along the length of the drill stringelectrode section 57 simply by measuring the depth of the drill string.

The data cable in the wireline is connected at its lower end to aninstrument package 94 that is secured to the distal end of the wireline,below the electrode 92, with the wireline being long enough to locatethis package centrally within the nonmagnetic sub 84, and is connectedat its upper end to suitable control circuitry at the surface, such as acomputer 42 (FIG. 1).

Because the MWD measuring instrument 88, which is conventionally used tomeasure the magnetic fields generated in the earth by current flowing inthe target, as discussed above, is located above the hydraulic drillingmotor 64 in the illustrated embodiment, magnetic field and othermeasurements needed for determining the distance and direction to thetarget and for guiding the drilling operation are normally received at adistance of 10 to 20 meters behind (or above) the actual location of thedrill bit 68 that is being controlled. When target magnetic fieldvectors are determined at this distance above the bit, inaccuracies inthe control of the drill bit occur, and these can produce unacceptableerrors when the relief well is approaching the target.

The foregoing problem is overcome, in accordance with the presentinvention, by providing magnetic field and other sensors in a drill bitsensor instrument 102 mounted on the drill bit sub 70. A schematicdiagram of a suitable sensor instrument 102 is illustrated in FIG. 4,while an enlarged view of the sensor instrument is illustrateddiagrammatically in FIGS. 5 and 6. As shown in these Figures, theinstrument 102 incorporates a three-part AC magnetometer having sensorcomponents 103, 104, and 105 for measuring x, y and z vector componentsof a varying electromagnetic field H that is generated by current flowon a target such as a well casing. These magnetometer sensor componentsmay be constructed, for example, using coils surrounding U-shaped coresin accordance with the teachings of U.S. Pat. No. 4,502,010, mentionedabove. The instrument 102 also contains an orientation package 106 fordetermining the orientation of the AC magnetometers. For this purpose,package 106 may contain two-component or three-component accelerometers,a one-component gyroscope and a 3-component earth field DC magnetometerfor detecting vector components of perturbations in the Earth's field.These apparent Earth field measurements can also be used to determineany static magnetic fields generated by the target well. From thesevarious measurements the relative location of the target well withrespect to the drill bit sensor instrument 102, and thus with respect tothe drill 68, can be determined using well known methods of magneticfield analysis.

The drill bit instrument 102 also incorporates an AC voltage amplifier107, whose input terminals are connected to measure the voltagedifference between the outer sleeve 116 and the drilling motor which isconnected to the drill bit instrument body 70. This AC voltagedifference gives the polarity and magnitude of the electric field in thenearby Earth and thus provides a direct measurement of the sense of theAC current flow on the target well relative to the measured AC magneticfield vectors Hx1, Hx2, Hy1, Hy2, and Hz. With a symmetric AC currentwaveform on the target well there may be some ambiguity in the sense ofthe current flow which is removed by this measurement. This signambiguity can also be determined by including an even time harmoniccomponent to the AC current injected into the formations. In many casesthis ambiguity also can be removed by well known, indirect means such asby noting the character of measurements at other nearby depths.

The magnetometer components 103, 104, and 105, the orientation package106, and the AC amplifier 107 are connected to a down-hole controlcomputer 108 in the instrument 102 for preliminary processing ofreceived data and the computer is, in turn, connected by way of suitablecommunications telemetry, such as a transmit/receive solenoid coil 110,for wirelessly transmitting data to the wireline instrument package 94,illustrated in FIG. 3, via the communications link 111. Although thelink provided by such solenoids have a limited communication range whenused underground, sufficient power is provided by a battery pack 112 inthe drill sub instrument 102 to provide reliable data communicationbetween the instrument 102 and the wireline instrument 94, which isnormally less than about 30 meters distant. In order to preserve power,the computer 108 contains control circuitry that responds to the absenceor presence of output signals from the magnetometers 103, 104 and 105,in response to magnetic fields generated in the target, to turn theinstrument off when it is not being used, and on when field measurementsare to be made.

As illustrated in FIGS. 5 and 6, the drill bit sensor instrument 102 ismounted in a cavity 113 in the drill bit sub 70. The sub 70 surrounds anaxial opening 114 through which the drill motor shaft 66 extends todrive the drill bit 68, with the communication solenoid 110 beingwrapped around the inner wall of cavity 113 to surround the axialopening 114. Cavity 113 is covered by a stainless steel cover tube 116that is secured in place on the drill bit sub by a suitable insulatingadhesive. The drill bit sub 70 is threaded at its upper end 118 toengage the threaded lower end of housing 64, while the lower end 120 ofsub 70 is threaded to receive the drill bit.

The main wireline instrument 94 carried at the end of the wireline 90 isillustrated schematically in FIG. 7 as incorporating a control computer124 connected to suitable communications telemetry such as anelectromagnetic transmit/receive communication circuit 126, which mayinclude a solenoid 127, that is similar to the coil 110 illustrated inFIG. 6, for receiving data from the drill bit instrument 102, and,optionally, for controlling the operation of instrument 102. Thecomputer 124 also is connected to computer 42 at the surface bytelemetry circuit 128 via data cable 129 carried by wireline 90.

In accordance with the method of the present invention, drilling of arelief or other borehole is carried out, for the most part, in the knownmanner illustrated in FIG. 1, but using the drill string structuredescribed with respect to FIGS. 3, 4, 5 and 6. In the illustrated formof the invention, drilling fluid flows down through the center of thedrill string 50 to provide driving power for the bent sub hydraulicdrilling motor 62, and the direction of drilling is controlled byturning the drill string so that the borehole will be drilled in thedirection faced by the bent housing and the drill bit. It will beunderstood, however, that other conventional directional drillingassemblies may be used to drive the drill bit and to steer the drillingof the borehole. The drill bit instrument 102 in sub 70 rotates with thedrill bit, but is turned off during drilling, while the MWD system 88controls the drilling operation in known manner.

In order to precisely measure the distance and direction from the drillbit to the target to permit accurate guidance of further drilling, thedrilling is stopped, and the wireline 90, incorporating at least thefirst electrode 92 and its instrument package 94, is lowered down thecenter of the drill string. If necessary, the drilling fluid can bepumped to assist in carrying the wireline down the drill string. Theinstrument 94 is lowered into the nonmagnetic sub 84 so that thewireline electrode 92 is positioned in its corresponding drill pipeelectrode section 57. The electrodes are in effective electrical contactwith each other, so that when power is supplied from source 24, thedrill pipe section 57 acts as an injection electrode for injectingelectrical current into the earth surrounding the borehole. Although thepower supply 24 is preferably a low-frequency AC source, as describedabove, a DC source may be used if desired, with down hole switchingproviding alternating or pulsed current to the surrounding earthformations. The pipe section 57 produces current flow in the earth bycontacting the earth directly or through the drilling fluid that flowsup-hole around the outside of the drill string from the region of thedrill bit to the surface.

As noted in FIG. 3, standard steel drill pipe sections, such as sections57, 58 and 59, are usually 10-meter long threaded pipes, the nonmagneticpipe section 84 is also nominally 10 meters long, and the threaded,electrically insulating subs, such as subs 71, 72 and 82, are each about1 meter in length. The standard hydraulic motor housing 64 is nominally10 meters long, and the rotating drill bit shaft 66 with its bit 68 andinstrument sub 70 is about 1.5 meters long. When the drill string isassembled, then, the relative locations of the nonmagnetic sub 84 andthe electrode pipe sections 57 are known, as is the total length of thedrill string, so that the wire line can easily be positioned in thedrill string with the wireline electrode 92 properly located centrallyalong the length of its corresponding drill pipe electrode sections. Thedistance between the electrode 92 and the drill head 68 will depend onthe number of pipe sections inserted between sections 59 and 60 (in FIG.3), but desirably this distance will be approximately 70 meters.

After the wireline 90 is positioned in the drill string, electrode 92 isenergized, as illustrated in FIG. 8, to inject several amperes ofcurrent 130 having, for example, a frequency of about 1 to 20 Hertz,into the earth formation 18 surrounding the target well 12 and therelief well 52. As in the prior art described with respect to FIGS. 1and 2, the injected current flow through the earth eventually returns tothe ground point 26, with part of this alternating current flowingthrough the conductive path of least resistance in target well 12, asillustrated at 132 in FIG. 8. The target current 132 has the amplitudevs. depth characteristic illustrated by FIG. 2, with the maximum currenton the target occurring at a depth that is approximately midway betweenthe electrode 92 and the earth's surface, and at a similar distancebelow the level of the electrode. The current 132 produces acorresponding target magnetic field 136 around target well 12, as wasdescribed with respect to FIG. 1, which field is detectable by the drillbit instrument 102. At the drill bit, target field vectors and othermeasurements are processed and transmitted electromagnetically to thewireline instrument package 94 for retransmission to the computer 42 atthe earth's surface. Since this target field is measured at the drillbit, the calculations made by computer 42 of the distance and directionfrom the bit to the target are more accurate than would be possible atthe depth of the wireline instrument package 94 or with measurementsmade at the conventional MWD instrument located above the motor 64.

Another embodiment of the invention, which may be desirable in somecircumstances, is illustrated in FIG. 9, wherein components similar tothose of FIG. 3 are similarly numbered. In this embodiment, a secondelectrically isolated steel pipe section, such as a drill pipe section130 connected between upper and lower insulating subs 132 and 134, maybe provided in the drill string to form a second drill string electrode.The section 130 is illustrated as being spaced below drill pipe section58 and above section 59 so that it is suitably spaced from, andelectrically insulated from, both the drill pipe electrode 92 and theinstrument package 94. When such a second electrode pipe section isprovided, the wireline will carry a second electrode 136 which will bepositioned within the second drill pipe electrode section 130 when thewireline is inserted in the drill string. The wireline electrode 136will then make electrical contact with the interior of the pipeelectrode 130 through physical contact and/or via conductive drillingfluid in the drill string. This second wireline electrode 136 isconnected to the AC source 24 through the wireline power cable, asdescribed for electrode 92. This two-electrode system operates in amanner similar to that described above.

In still another embodiment of the invention, diagrammaticallyillustrated in FIG. 10, a drill string 140 in a borehole being drilledincorporates at least one, and preferably two spaced electrode sections142 and 144 that are electrically isolated from the remainder of thedrill string by respective electrically insulating subs 146, 148 and150, 152, in the manner discussed above with respect to FIGS. 3 and 9.The drill string includes conventional drill string sections above theelectrode section 142, as illustrated at 154, and between the electrodesections 142 and 144, as illustrated at 156, the number of sections at156 being sufficient to space the electrode sections 142 and 144 apartby about 150 meters. Additional conventional drill pipe sections areprovided at 158 between the drill pipe electrode 144 and a nonmagneticdrill pipe section 160 to space the lower electrode about 70 metersabove a standard directional drilling assembly such as the illustratedconventional bent sub hydraulic drilling motor 162 and drill head, orsub 164 at the distal end of the drill string, in the manner describedabove with respect to FIGS. 3 and 9. As before, the nonmagnetic drillpipe section 160 incorporates a conventional MWD instrument 166 mountedabove the hydraulic drive motor, and the drill head, or sub 164, whichis mounted on the bent sub 162 and is driven by the motor, carries themagnetic field sensor instrument 70 with instrumentation 102, describedabove with respect to FIG. 4.

In the embodiment of FIG. 10, the need for a wireline that can belowered down the center of the drill stem is eliminated. In its placeare communications electronics such as a transmit/receive (TR) package170 located in the nonmagnetic section 160 of the drill string. Asillustrated in FIG. 11, the TR electronics package 170 preferablyincludes a solenoid 172 and TR circuitry 174 for communicating with thedrill head instrument 70 via an electromagnetic link illustrated at 176in FIG. 10. The package 170 also incorporates an electrode power supply178, such as a battery pack and a suitable inverter, an alternator, orboth, with the power supply being connected through suitable cables 180and 182 to the drill string electrodes 142 and 144, respectively. Thepower supply provides AC current 183 to the electrodes for injectioninto the formations surrounding the borehole, as illustrated at 184, toprovide a target current flow and its corresponding target magneticfield 185. The drill head sensor instrument 102 includes sensors 103,104, and 105 for detecting the target magnetic field and the transmitterand receiver 110 for communicating data corresponding to the detectedmagnetic field to the transmit/receive package 170. The TR circuitry 174in package 170 is connected by way of data cable 186 to the standard MWDinstrument 166 that is also located in the nonmagnetic section. The MWDinstrument then transmits this data to surface receivers in conventionalmanner, as by way of pressure pulses in the drilling fluid. The MWDinstrument also receives control signals from the surface, again in aconventional manner, which signals are communicated to the drill bitinstrument 102 by way of the TR package 170 to control drillingoperations in conventional manner, as well as to initiate the injectionof current and the measurement of the resulting magnetic fields whendrilling has been stopped, and to enable data transfer betweeninstrumentation at the surface and the drill head sensor instrument.

As has been described above, the current injected into the Earthformations surrounding the drill string in the borehole being drilled isa low-frequency alternating current with an amplitude of several amps.However, in another form of the invention, the injection currentsupplied to the drill string electrode configurations from the powersupplies described above may be a transient pulsed signal, asillustrated by the square wave 190 in FIG. 12( a), which starts at timet₀ and stops at time t₁ and has a repetition rate of multiple times persecond. The current to be injected into the surrounding formations, forexample current 96 in FIG. 8 or current 183 in FIG. 10, may be severalamperes in amplitude, and produces the corresponding formation currents130 (FIG. 8) or 184 (FIG. 10) and target current 132, also illustratedin FIG. 8. The current path in each of the two-electrode configurationsof FIGS. 9 and 10, forms an inductive loop and causes the target wellcurrent (132 in FIG. 8) to increase from t₀ to t₁ as illustrated bycurve 192 in FIG. 12( b), and at time t₁, when the square wave pulseends, the target well current begins to decay, as illustrated by curve194. This “L/R” time decay, which is caused by eddy currents primarilyin the target but also in the surrounding earth formations, is about 1.5milliseconds for typical earth formations in an environment such as anoil field and is illustrated in FIG. 13 as a decay current 196 in atarget such as well 12. This decay current in the target well decreasesin amplitude over time, as illustrated by curves 198 in FIG. 14, flowsinto the surrounding formation, as indicated at 200, and produces adecay current magnetic field 202 surrounding the target well, asillustrated in FIG. 13. The vector components of this target magneticfield 192 are detected by the magnetometers 103, 104, and 105 in thedrill bit sensor instrument 102, with the AC field measurements beingmade at 102 during the decay period t₁ to t₂. The measurements madeduring this time are free of the interference that is caused by magneticfields generated by the drilling well currents 96 or 183, by relatedleakage currents, and by formation effects, thereby providing moreaccurate and reliable measurements for use in determining the locationof the target.

The AC field measurement data, the AC voltage data, and the orientationmeasurement data obtained by the drill bit sensor instrument 102 arepartially processed by control computer 108 in sensor instrument 102 andare sent by the two-way electromagnetic communication package 110 to themain instrument package 94 (FIGS. 3 and 9) on the wireline, or to thepackage 170 in the system of FIG. 10. This data is then transmittedup-hole by way of the data cable on wireline 90 in the embodiments ofFIGS. 3 and 9, or by the MWD instrument in the embodiment of FIG. 10, tothe surface computer 42 for processing. If desired, this computer maythen send appropriate control signals by way of wireline 90 to theinstrument 94 or by way of the mud pulses, for example, to the MWDinstrument 166, which may transmit these control signalselectromagnetically to the drill bit control computer 108 to turn thesensor instrument 102 on or off. Alternatively, the sensor instrumentmay be turned on or off automatically when it detects, or no longerdetects, a target magnetic field. When the data collection has beencompleted, drilling operations resume, using directional controlsderived from the distance and direction data obtained from the downholemagnetic field and orientation data at the bit instrument 102 to guidefurther drilling via the MWD instrument.

Although the present invention has been described in terms of preferredembodiments, it will be understood that numerous modifications andvariations in the apparatus described herein may be made withoutdeparting from the true spirit and scope of the invention as set out inthe following claims.

1. Apparatus for target proximity detection from a borehole beingdrilled, comprising: a drill string having multiple drill pipe sectionsconnected end-to-end and carrying a drill bit; at least one of saiddrill pipe sections being electrically conductive to provide a drillpipe electrode section; at least one electrically insulating drill pipesub electrically isolating said electrode section from adjacent drillpipe sections; a power supply in electrical communication with said atleast one drill pipe electrode section and energizable to inject atime-varying current into Earth formations surrounding said borehole; adrill bit instrument at said drill bit for detecting magnetic fieldsproduced by said injected current; and communication electronics locatedin said drill string for establishing communication between said drillbit instrument and surface instrumentation for sending detected magneticfield data to said surface instrumentation.
 2. The apparatus of claim 1,further including: a wireline locatable within said drill string, saidwireline incorporating a wireline electrode connected to said powersupply for electrical communication with said drill string electrode forinjection of said time-varying current into Earth formations; andwherein said wireline carries a part of said communication electronicsfor communication of detected magnetic field data to said surfaceinstrumentation.
 3. The apparatus of claim 2, wherein said drill stringfurther includes: first and second electrically conductive drill pipeelectrode sections spaced apart along the length of the drill string andelectrically isolated from each other and from adjacent drill pipesections by electrically insulating drill pipe subs; and first andsecond electrodes on said wireline, said wireline electrodes beingspaced to be located within and in electrical communication withrespective first and second drill pipe electrode sections when saidwireline is inserted in said drill string.
 4. The apparatus of claim 1,wherein said drill bit instrument incorporates first magnetic fieldsensors for detecting vector components of time-varying magnetic fieldscharacteristic of said injected current and second magnetic fieldsensors for detecting static magnetic field vectors.
 5. The apparatus ofclaim 4, wherein said communication electronics includes drill bittelemetry in said drill bit instrument and wireline telemetry carried bysaid wireline.
 6. The apparatus of claim 1, wherein said drill stringfurther includes first and second electrically conductive drill pipeelectrode sections spaced apart along the length of the drill string andelectrically isolated from each other and from adjacent drill pipesections by electrically insulating drill pipe subs; and wherein saidpower supply is located in said drill string and is electricallyconnected to said first and second drill pipe electrode sections.
 7. Theapparatus of claim 6, wherein said power supply is a battery.
 8. Theapparatus of claim 1, further including a directional drilling assemblyconnected at a distal end of said drill string between said drill pipesections and said drill bit to drive said drill bit.
 9. The apparatus ofclaim 8, further including: a wireline locatable within said drillstring, said wireline carrying at least one wireline electrode forelectrical connection to said at least one drill string electrode andcarrying said communication electronics for location within anon-magnetic drill string section above said directional drillingassembly, said drill bit being located below said directional drillingassembly; magnetic field sensors mounted in said drill bit instrumentfor detecting said varying magnetic fields; an orientation packagelocated in said drill bit instrument; wherein said communicationelectronics includes a transmitter in said drill bit instrument fortransmitting data from said magnetic field sensors and said orientationpackage to said surface instrumentation.
 10. The apparatus of claim 9,wherein said communication electronics further includes a receivercarried by said wireline for receiving said data and retransmitting itto said surface instrumentation.
 11. The apparatus of claim 9, whereinsaid drill bit instrument is mounted in a housing at said drill bit, theapparatus further including an AC amplifier mounted in said drill bitinstrument for measuring electric fields between said directionaldrilling assembly and said housing to resolve sign ambiguity of detectedmagnetic fields.
 12. Apparatus for deep well measurements in a boreholebeing drilled to determine the distance and direction from the boreholeto a target, comprising: a drill string in said borehole, the drillstring having multiple drill pipe sections connected end-to-end andcarrying at a distal end a directional drilling assembly and a drill bitsub; at least one of said drill pipe sections being electricallyconductive to provide an electrode section; at least one electricallyinsulating drill pipe sub electrically isolating said electrode sectionfrom adjacent drill pipe sections; a non-magnetic drill string sectionbetween said drilling motor and said at least one drill pipe electrodesection; a power supply connectable to said drill pipe electrode sectionto inject current into the earth surrounding said borehole and toproduce a corresponding current flow in said target; a drill bitinstrument at said drill bit sub and including first magnetic fieldsensors for detecting vector components of time-varying magnetic fieldsproduced by said current flow in said target; communication electronicswithin said nonmagnetic drill pipe section; and telemetry fortransmitting data corresponding to said vector components from saiddrill bit instrument to said communication electronics.
 13. Theapparatus of claim 12, wherein said drill string includes twoelectrically conductive drill pipe electrode sections spaced apart alongthe length of the drill string and electrically isolated from each otherand from adjacent drill pipe sections by electrically insulating drillpipe subs, and wherein said power supply is connected to both of saidelectrically conductive drill pipe sections.
 14. The apparatus of claim13, wherein said power supply is located at the Earth's surface and isconnected to said drill pipe electrode sections by a wireline insertableinto said drill string, the wireline incorporating a power cable and twowireline electrodes, each wireline electrode being locatable within acorresponding drill pipe electrode section and connected through saidpower cable to said power supply; wherein said wireline carries saidcommunication electronics; and wherein said wireline incorporates a datacable connected to said communication electronics for transmitting datato surface instrumentation.
 15. The apparatus of claim 13, wherein saidpower supply is located in said drill string and is connected to saiddrill pipe electrode sections; the apparatus further including ameasurement while drilling (MWD) instrument in said non-magnetic drillstring section and connected to said communication electronics fortransmitting data received by said communication electronics toinstrumentation at the Earth's surface.
 16. A method for deep wellmeasurements in a borehole being drilled for determining the distanceand direction from the borehole to a target, comprising: locating adrill string in said borehole, the drill string having multiple drillpipe sections connected end-to-end and carrying at a distal end adirectional drilling assembly and a drill bit sub; providing at leastone electrically conductive drill pipe electrode section in said drillstring; electrically isolating said electrode section from adjacentdrill pipe sections; energizing said drill pipe electrode section toinject a time-variable current into the earth surrounding said boreholeand to produce a corresponding time-variable current flow in saidtarget; detecting vector components of time-varying magnetic fieldsproduced by current flow in said target at a drill bit instrument atsaid drill bit sub; transmitting data corresponding to said time-varyingmagnetic field to a communication instrument package located within anonmagnetic drill pipe section located above said drive motor;transmitting said data from said communication instrument package tosurface instrumentation; and determining from said data the distance anddirection from said drill bit sub to said target.
 17. The method ofclaim 16, further including detecting vectors of the earth's magneticfield and gravity at said bit instrument to obtain Earth fieldperturbation data; and transmitting said Earth field perturbation datato said communication instrument package.
 18. The method of claim 16,further including supplying said time-variable current to twospaced-apart drill stem electrodes to produce corresponding time-varyingcurrents in the earth surrounding the borehole and in said target. 19.The method of claim 18, wherein supplying said time-variable currentcomprises supplying a transient pulsed current having a decay period toproduce a corresponding decaying current having a decay period in saidtarget, the decaying target current producing a correspondingcharacteristic decaying target magnetic field.
 20. The method of claim19, further including detecting vectors of said time-varying magneticfields produced by current flow in said target during the decay periodof said target current.
 21. The method of claim 20, further includingtransmitting said vector data from said communication instrument packageto a computer for determining the location of said target with respectto said drill bit sub.
 22. The method of claim 16, further includingresolving the sign ambiguity of detected magnetic fields.